1. Field of the Invention
The present invention relates to electroencephalographic signal monitoring and, more particularly, to the monitoring of conditions reflected in electroencephalographic signals through an indicator based on such signals.
Situations occur in which the cerebral perfusion pressure can be insufficient to the point where parts of the human brain have a supply of blood so insufficient as to put a patient at risk of neurologic damage. Such situations can arise, for instance, in surgeries utilizing cardiopulmonary bypass machines, i.e. a heart-lung machine. Such a machine serves as a pump and oxygenator to supply oxygenated blood in place of the heart and lungs during such surgery, for instance, open-heart surgery. Other surgery situations which could interfere with brain blood flow are carotid endarterectomies and cerebral aneurysm repairs.
The sufficiency of the cerebral perfusion pressure is reflected to a considerable extent in the electroencephalographic signals obtained from such a subject or patient. Electroencephalographic signals are minute electrical signals produced in the brain. An electroencephalograph measures the electrical potential at the surface of the scalp of the patient's head by the use of electrodes pasted to the surface of the scalp typically at one or more of the standard positions adopted by the International Federation of Electroencephalography in what is called the 10/20 system.
There can be as many as 20 electrodes provided in this manner which are connected to electroencephalographic equipment to provide indications of the potentials measured. These potentials are typically in the range of 1 to 200 .mu.V.
The electrical potentials measured and represented as electroencephalographic signals have been found to provide a relatively sensitive indication of whether adequate blood flow is being maintained in the brain. Cerebral perfusion pressure must not drop too far for too long a period of time, or a cerebral disorder may result because of the lack of oxygen to the brain or because of disturbances in the brain's cellular metabolism. The necessary blood flow in the brain, however, varies from individual to individual, depending on the characteristics of the patient including age, metabolism and the like. Thus, a convenient indicator from a set of electroencephalographic signals being monitored would be desirable even though there are no absolute criteria.